Smoothening of the aluminum surfaces during the growth of barrier anodic oxide films has been investigated by transmission electron microscopy of the stripped films and ultramicrotomed sections. It was found that three factors are responsible for the smoothening: (1) the anodic oxide films are amorphous, so that they can follow the continuously changing and reducing aluminum surfaces; (2) the oxide films should be of uniform thickness in the direction perpendicular to the local metal surfaces as required by the oxide growth kinetics; and (3) part of the oxide growth occurs at the oxide/electrolyte interface by the outward diffusion of Al³⁺ ions.